The present invention relates to an apparatus for forming a silicon oxide film and a method of forming a silicon oxide film.
For example, in production of a MOS type semiconductor device, it is required to form a gate oxide film composed of a silicon oxide film on a surface of a silicon semiconductor substrate. In production of a thin film transistor (TFT), it is also required to form a gate oxide film composed of a silicon oxide film on a surface of a silicon layer formed on an insulation substrate. It can safely be said that reliability of the semiconductor devices depends upon these silicon oxide films. The silicon oxide films are therefore constantly required to have high dielectric breakdown durability and long-term reliability.
With a decrease in thickness of a gate oxide film and an increase in diameter of a substrate, an apparatus for forming a silicon oxide film has been being converted from a horizontal-type apparatus in which a process chamber (oxidation chamber) formed of quartz extends in a horizontal direction to a vertical-type apparatus in which a process chamber extends in a vertical direction. The reason therefor is as follows. Not only the vertical-type apparatus for forming a silicon oxide film can easily cope with an increase in a diameter of a substrate as compared with the horizontal-type apparatus, but also the vertical-type apparatus can serve to decrease formation of a layer of silicon oxide (to be referred to as "natural oxide" hereinafter) caused by atmosphere taken into the process chamber of the vertical-type apparatus during transfer of silicon semiconductor substrates into the process chamber. However, even the use of the vertical-type apparatus for forming a silicon oxide film results in the formation of a natural oxide having a thickness of approximately 2 nm on the surface of the silicon semiconductor substrate. The natural oxide contains a large amount of impurities derived from atmosphere, and the presence of the natural oxide is not at all negligible when a gate oxide is decreased in thickness. There have been therefore proposed methods for preventing the formation of the natural oxide to the lowest level possible, such as (1) a method in which a nitrogen gas atmosphere is formed in a substrate transfer portion provided in a vertical-type apparatus by flowing a large volume of nitrogen gas (nitrogen gas purge method), and (2) a method in which a substrate transfer portion is vacuumed and then nitrogen gas or the like is introduced into the substrate transfer portion to discharge atmosphere (vacuum loadlock method).
Thereafter, in a state where an inert gas atmosphere is formed in the process chamber (oxidation chamber), silicon semiconductor substrates are brought into the process chamber (oxidation chamber). Then, an atmosphere of the process chamber (oxidation chamber) is replaced with an oxidative atmosphere and the silicon semiconductor substrates are thermally oxidized to form gate oxide films. For the formation of the gate oxide film, there is generally employed a method in which high-purity water vapor is introduced into the process chamber maintained at a high temperature to thermally oxidize the surface of the silicon semiconductor substrate (wet oxidation method). In this method, a gate oxide film having high electric reliability can be obtained as compared with a method in which the surface of the silicon semiconductor substrate is oxidized with high-purity dry oxygen gas (dry oxidation method). Included in the above wet oxidation method is a pyrogenic oxidation method (also called "hydrogen gas combustion oxidation method or wet oxidation") in which hydrogen gas is mixed with oxygen gas at a high temperature and is combusted and the so-generated water vapor is used. The pyrogenic oxidation method is widely used. In the pyrogenic oxidation method, generally, oxygen gas is supplied into a combustion chamber which is disposed outside the process chamber (oxidation chamber) and which interior is maintained at 700 to 900.degree. C., and then hydrogen gas is supplied into the combustion chamber to combust the hydrogen gas at a high temperature. The so-obtained water vapor is used as oxidizing species.
FIG. 21 shows a schematic view of a vertical-type apparatus for forming a silicon oxide film by the pyrogenic oxidation method. The vertical-type apparatus comprises a double-tubular structured process chamber 10 made of quartz and held perpendicularly, a water vapor inlet port 12 for introducing water vapor and the like into the process chamber 10, a gas exhaust portion 13 for exhausting the gas from the process chamber 10, a heater 14 for maintaining the interior of the process chamber 10 at a predetermined ambient temperature through a cylindrical heat equalizer tube 16 made of SiC, a substrate transfer portion 20, a gas introducing portion 21 for introducing nitrogen gas into the substrate transfer portion 20, a gas exhaust portion 22 for exhausting the gas from the substrate transfer portion 20, a shutter 15 for partitioning the process chamber 10 and the substrate transfer portion 20, and an elevator unit 23 for bringing silicon semiconductor substrates into and out of the process chamber 10.
A base portion 26 is attached to the elevator unit 23, and a heat insulation member 25 is disposed on the base portion 26. Further, onto the heat insulation member 25 is attached a substrate receiving member 24 made of quartz or SiC for receiving silicon semiconductor substrates therein. A sealing member 27 formed of, for example, an "O-ring" is attached to a marginal portion of the upper surface of the base portion 26, and when the substrate receiving member 24 is brought into the process chamber 10, the lower portion of the process chamber 10 is sealed with the base portion 26 (see FIG. 22). The base portion 26 is structured so as to flow coolant inside.
The heat insulation member 25 is also called a heat-retaining cylinder or a heat barrier, and generally, it is a hollow and cylindrical member having its top and bottom surfaces closed and being formed of quartz, and it has a structure in which the hollow portion is filled with glass fiber. Further, a piping 17 for flowing coolant is disposed outside the process chamber 10 and near the heat insulation member 25. In the above structure, damage of the sealing member 27 caused by radiation heat directly conducted to the base portion 26 in the process chamber 10, can be prevented and malfunction of the elevator unit 23 can be reliably prevented.
Hydrogen gas supplied to a combustion chamber 30 is mixed with oxygen gas at a high temperature and combusted in the combustion chamber 30 to generate water vapor. The water vapor is introduced into the process chamber 10 through a piping 31, a gas flow passage 11 and a water vapor inlet port 12. The gas flow passage 11 corresponds to a space between an inner wall and an outer wall of the double-tubular structured process chamber 10.
A conventional method of forming a silicon oxide film with a conventional apparatus having the above structure will be outlined with reference to FIGS. 23 to 25 hereinafter. [Step-10]
First, nitrogen gas is introduced into the process chamber 10 through a piping 32, the combustion chamber 30, the piping 31, the gas flow passage 11 and the water vapor inlet port 12 to form a nitrogen atmosphere in the process chamber 10, and the ambient temperature in the process chamber 10 is maintained at 700 to 750.degree. C. with the heater 14 through the heat equalizer tube 16. The purpose in maintaining the ambient temperature in the process chamber 10 at the above temperature range is to decrease thermal shock which silicone semiconductor substrates 50 suffer when the silicon semiconductor substrates 50 are transferred into the process chamber 10. In this state, the shutter 15 is kept closed. The substrate transfer portion 20 is in a state where it is open to atmosphere. Further, the piping 17 has coolant flowing. [Step-20]
Silicon semiconductor substrates 50 are transferred into the substrate transfer portion 20, and placed in the substrate receiving member 24. After the transfer of the silicon semiconductor substrates 50 into the substrate transfer portion 20 is completed, a door (not shown) is closed. Then, nitrogen gas is introduced into the substrate transfer portion 20 through the gas introducing portion 21 and is exhausted through the gas exhaust portion 22, to form a nitrogen gas atmosphere in the substrate transfer portion 20 (see FIG. 23A). The base portion 26 has coolant flowing inside. [Step-30]
When a sufficient nitrogen gas atmosphere is formed in the substrate transfer portion 20, the shutter 15 is opened (see FIG. 23B), and the elevator unit 23 is actuated to elevate the substrate receiving member 24 approximately at a rate of 50 mm/minute, whereby the silicon semiconductor substrates 50 are transferred into the process chamber 10 (see FIG. 24A). When the elevator unit 23 reaches its uppermost position, the sealing member 27 comes into contact with the bottom of the process chamber 10, and the process chamber 10 is closed with the base portion 26, whereby the process chamber 10 and the substrate transfer portion 20 are no longer communicated with each other (see FIG. 22). [Step-40]
Then, after the ambient temperature in the process chamber 10 is fully stabilized, the ambient temperature is increased up to 800 to 900.degree. C. (see FIG. 24B). Oxygen gas and hydrogen gas are supplied to the combustion chamber 30 through the pipings 32 and 33, and the hydrogen gas is mixed with the oxygen gas at a high temperature and combusted in the combustion chamber 30 to generate water vapor. The water vapor is introduced into the process chamber 10 through the piping 31, the gas flow passage 11 and the water vapor inlet port 12, and is exhausted through the gas exhaust portion 13 (see FIG. 25A), whereby a silicon oxide film is formed on the surface of each silicon semiconductor substrate 50. The temperature in the combustion chamber 30 is maintained at 700 to 900.degree. C., for example, with a heater (not shown). [Step-50]
After the silicon oxide films having a predetermined thickness are formed, the supply of the water vapor into the process chamber 10 is terminated, and an inert gas atmosphere such as a nitrogen gas atmosphere is formed in the process chamber 10. Then, the ambient temperature in the process chamber 10 is decreased to 700 to 750.degree. C. for decreasing thermal shock on the silicon semiconductor substrates 50 (see FIG. 25B). Then, after the ambient temperature in the process chamber 10 is stabilized, the elevator unit 23 is actuated to lower the substrate receiving member 24, and the silicon semiconductor substrates 50 are transferred out of the substrate transfer portion 20.
Since coolant is continuously flowed in the piping 17 and further since coolant is continuously flowed inside the base portion 26, a large temperature gradient is caused between the ambient temperature in a process chamber area where the substrate receiving member 24 is positioned and the heat insulation member 25 when the ambient temperature in the process chamber 10 is increased, for example, to 850.degree. C. in Step-40, and the heat insulation member 25 has a surface (outer surface) temperature of 150 to 200.degree. C. or lower although it differs depending upon an apparatus for forming a silicon oxide film.
In the conventional method of forming a silicon oxide film, the ambient temperature in the process chamber 10 is decreased to 700 to 750.degree. C. and then the silicon semiconductor substrates 50 are transferred out of the process chamber 10 in Step-50. Therefore, even if dew (water drops) is formed on the surface of the heat insulation member 25 in the process of forming a silicon oxide film, the dew on the heat insulation member 25 is evaporated since the process chamber 10 is maintained to have an inert gas atmosphere at 700 to 750.degree. C. for a certain period of time in Step-50.
In recent years, the thickness of a gate oxide film is decreased for higher integration of an LSI, and with a decrease in the above thickness, the thermal oxidation temperature of the silicon semiconductor substrates is decreased. That is because the time period for the oxidation needs to be extremely decreased or shortened at a conventional thermal oxidation temperature of 800 to 900.degree. C.
Meanwhile, it has come to be known that when the thermal oxidation temperature is set at a low temperature (for example, 700 to 750.degree. C. or lower), the heat insulation member 25 has a surface temperature of less than 100.degree. C. so that dew (water drops) is formed on the surface of the heat insulation member 25 in the step of forming a silicon oxide film. When the silicon semiconductor substrates 50 are transferred out of the process chamber 10 with the dew on the surface of the heat insulation member 25, a metal portion or a metal member of the elevator unit 23 may be corroded. When the metal portion or the metal member is corroded, not only the elevator unit 23 malfunctions, but also a corroded portion can be a source which gives metal impurities. When metal impurities are therefore included in the process chamber 10, characteristics of the silicon oxide films are deteriorated. Further, since the silicon semiconductor substrates 50 have a temperature of hundreds degree C. (.degree.C.) immediately after they are transferred to the substrate transfer portion 20, the dew on the surface of the heat insulation member 25 is evaporated to generate water vapor. When the water vapor comes into contact with the silicon semiconductor substrates 50, the silicon semiconductor substrates 50 suffer stains similar to water marks on their surfaces, which results in in-plane non-uniformity of the silicon oxide films.